The marine controlled-source electromagnetic (“CSEM”) exploration method uses man-made sources to generate electromagnetic (EM) waves and deploys receivers on the seafloor to record electromagnetic signals. The recorded electromagnetic signals are analyzed to infer subsea floor structures and/or determine nature of particular structures such as reservoirs. FIG. 1 illustrates a typical deployment of CSEM equipment, with an horizontal electric dipole (HED) source 11 towed by a vessel above the water bottom 12 on which receivers 13 are placed. This technology has been applied in hydrocarbon and mineral exploration, and also in other areas such as tectonic studies and environmental and geological engineering.
At present, receivers fall freely to the seafloor and therefore their orientations are unknown. Receiver orientations are required to determine the three-dimensional EM field vectors measured at receiver locations. The measured fields are then decomposed into components in preferred directions (for example, inline, crossline, and vertical) for analysis, inversion and interpretation. Effects on the decomposed components could be significant when the receiver cannot be oriented to those preferred directions because of inaccurate receiver orientations. Therefore the determination of receiver orientations could significantly affect data interpretation. The present invention provides a technique to determine receiver orientations.
In order to completely measure three-dimensional EM fields, receivers need be equipped with three mutually-perpendicular antennas for electric fields and three mutually-perpendicular magnetic sensors for magnetic fields. Three angles are necessary and sufficient to uniquely define the receiver orientations. These three angles establish the relationships between the measurement coordinates and receiver coordinates. A number of ways can be used to define the receiver orientations in the measurement coordinates. They are equivalent and can be converted one another. One way to define the receiver orientations is using azimuth and tilts for two horizontal channels (FIG. 2). In FIG. 2, (X, Y, Z) are assumed to be the measurement coordinates with X directed to the geodetic east, Y to the geodetic north, and Z upward. (X′″, Y′″, Z′″) are the receiver coordinates and the designed “east”, “north” and vertical channels. (X′, Y′, Z′) and (X″, Y″, Z″) are auxiliary coordinates to help transform coordinates between (X, Y, Z) and (X′″, Y′″, Z′″). X′ is the projection of X′″ on the horizontal plane XY, while Y″ is the projection of Y′″. With those setups, the receiver azimuth (α) is defined the angle between Y and Y′, the east channel tilt (β) is the angle between X′ and X′″, the north channel tilt (γ) is the angle between Y″ and Y′″.
At present, a number of methods are known for determining receiver orientations. One is to directly measure receiver orientations. Present direct measurement systems for marine CSEM receivers have reliability problems. Even with such a system available, an alternative method would be still necessary in some circumstances: for example, 1) when the direct measurement system is broken; 2) when the measurement system is not aligned with the directions of electrodes/magnetic sensors because of bending of long electric antennas on the seafloor.
Another method is polarization analysis, disclosed in Behrens, J. P. (2005), “The Detection of Electrical Anisotropy in 35 Ma Pacific Lithosphere: Results from a Marine Controlled-Source Electromagnetic Survey and Implications for Hydration of the Upper Mantle,” Ph.D. Thesis, University of California, San Diego (2005). Also see Constable and Cox, “Marine controlled source electromagnetic sounding 2: The PEGASUS experiment,” Journal of Geophysical Research 101, 5519-5530 (1996). The method is based on the fact that the EM field amplitude of the signal recorded by a receiver is maximized when the receiver antenna is in the direction of the transmitter (i.e. the major axis of the polarization ellipse) provided the transmitter is towed directly towards the receiver. Polarization analysis was the primary method used in early marine CSEM work to determine receiver azimuth in the subsequent data processing. The method requires at least one towline towed directly over each receiver. Receiver azimuth accuracy provided by this method is not very high. The average error in receiver azimuths is larger than 5 degrees for field data from a boat with a dynamic position system. It could be worse for a ship without a dynamic positioning system in rough weather conditions.
Behrens also proposed to use coherency and correlation in natural EM signals recorded by different receivers to determine relative azimuth. This method was developed for receivers without a directly over-towing towline to complement the polarization analysis. The method determines the relative azimuth angle between two receivers. In order to find the receiver azimuth, the method requires the azimuth of the reference receiver be known. Success in using this method is dependent on whether high quality natural signals are recorded by both receivers. Accuracy by this method is normally lower than by polarization analysis.
R. Mittet et al. used inversion to determine receiver azimuth in “E020: Inversion of SBL data acquired in shallow waters,” EAGE 66th Conference & Exhibition—Paris, France, Jun. 7-10 (2004). This method overcomes limitations on both the polarization analysis and the method of using natural EM signals. All three of these methods, though widely used, address only the receiver azimuth, but do not disclose how to determine receiver orientations uniquely, i.e. both the azimuth and the tilts of the two horizontal channels. The reasons for neglecting the receiver's other two angles are at least three in number. (1) Data interpretation is mainly focused on (and data measurement may be limited to) the inline (meaning along the tow direction) electric component, which is normally not affected much by the tilts if the seafloor is not very steep. (2) The vertical electric component is either not measured or is not fully utilized in data interpretation. (3) No reliable and accurate method is available to determine the receiver orientations. The two tilts are normally small (<10 degrees because the seafloor is normally flat. The three reasons are obviously not completely independent of each other.
In addition to the three reasons detailed above, the extent of possible impact on CSEM results of even small receiver tilt angles may not be appreciated. Effects of receiver orientations on the three electric components were simulated in the course of the present invention, and can be seen in FIGS. 3-5. The source and receiver geometry used in the model calculations that generated these three drawings is taken from an actual field survey. The resistivity model is a layered earth model with water depth of 125 m. The towline direction is 265.57 degrees from the geodetic north, clockwise. In the modelings, the receiver (azimuth, α in FIG. 2) misalignment (δα) with the towline (the inline direction) is 15 degrees, the inline antenna tilt (β) is up 5 degrees, and the crossline tilt (γ) is down 3 degrees. The modeling frequency is 0.25 Hz. In each of these three drawings, the solid line represents an aligned and level receiver, the circles a level receiver with δα=15°, the + symbols an aligned receiver with tilts of β=5° and γ=−3°, and the broken line a misaligned and tilted receiver. Compared with the ideal situation (a level receiver aligned with the towline, i.e. all three angles [δα, β and γ] zero), those figures show that while azimuth has much bigger effects on the two horizontal (inline and crossline) channels than do the tilt angles (especially on the cross component), the tilts have larger effects on the vertical component EZ. The effects can be significant, for example, about one order in magnitude for the cross and vertical components of this example (FIGS. 4 and 5). This example clearly demonstrates the importance of determining all three angles. Receiver azimuth alone cannot uniquely define the receiver orientations deployed on seafloor.
In summary, there is a need for a technique to determine receiver orientations that can be used without any limitations on transmitter and receiver geometry. The present invention satisfies this need.